JPH0646863B2 - Switching power supply circuit - Google Patents

Description

The present invention relates to a switching power supply, and more particularly to a simple and low noise circuit.

[Contents and problems of conventional technology]

In a switching power supply, it is effective to reduce the switching loss in the switching element and increase the conversion efficiency.One of the measures is to make the current waveform when the switching element conducts using a series resonance circuit into a sinusoidal waveform. The circuit method to do was adopted.

FIG. 4 shows an embodiment of the above-mentioned prior art, and FIG. 5 shows an example of its operation chart. The outline of the conventional technology will be described below with reference to these.

In FIGS. 4 and 5, the DC input voltage Vin is applied to the input terminals 1 and 2 and the output current is applied to the DC output terminals 3 and 4 by Io.
Is connected to the load L.

First, when the switching element SW5 is turned on, the first
The circuit current iL1 flowing through the diode D16 and the resonance inductor L18 starts to flow at the slope of Vin / L1. And time T1
When the output current Io becomes equal to the output current Io, the diode D16 is cut off, and the series resonance operation with the resonance capacitor C19 and the natural vibration period of ▲ 2π√ ▼ L1C1 is started, and the current flows through the resonance inductor 8 at time T3. The current iL1 reaches zero.

Therefore, if the switching element SW5 is turned off at this point, the circuit current iL1 at turn-on and turn-off can be suppressed and low-loss switching operation can be realized, and the output can be controlled by adjusting the repetition cycle thereof. Was there.

However, in the above-mentioned conventional technique, since the switching element SW5 is turned off when the circuit current iL1 becomes zero, it is necessary to provide a detector of the circuit current and a dedicated circuit function for turning it off. there were. Therefore, the circuit configuration is complicated, and there is a drawback that an inexpensive switching power supply cannot be provided with a simple configuration.

Further, when a method of fixing the ON period of the switching element 5 is adopted in order to configure a simple and inexpensive switching power supply with the above conventional configuration, a choke input smoothing circuit is provided in the resonance capacitor C19. Since the output circuits including the same are connected in parallel, the above-mentioned circuit current L1 once becomes zero as shown in FIG. 5, and then gradually starts to increase again.

Therefore, if the switching element is turned off at this point, the current flowing through the resonance inductor becomes discontinuous, and an excessive surge voltage is generated at both ends of it, causing an extremely large spike voltage to propagate to the input and output in each case. There were drawbacks, and its implementation was impossible.

[Object of the Invention]

SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above drawbacks and provide a switching power supply with a simple circuit configuration and low noise.

[Structure of Invention]

The present invention (1) uses a switching element to convert a DC input into alternating pulses directly or via a transformer, and rectifies and smoothes it to obtain a DC output. And a resonance inductor in series, and the inductor and the resonance capacitor are connected in series to form a unidirectional series resonance circuit, which is parallel to both ends of the resonance capacitor in the positive direction of the power supply voltage. Second
Is connected to the negative pole of the second diode to the negative pole of the power supply voltage and the smoothing circuit of the choke input by the choke coil and the smoothing capacitor to obtain a DC output, and By connecting the positive electrode to the input-side terminal of the resonance inductor and connecting the negative electrode of the diode to the negative electrode of the power supply, a current path that flows through the resonance inductor even after the pulse voltage disappears is provided. It is a switching power supply circuit.

(2) The switching power supply circuit according to claim 1, wherein the conduction time of the switching element is constant and the output voltage is constant by adjusting the repetition period.

[Explanation by Examples]

Next, a switching power supply using a series resonant circuit according to the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a circuit configuration of a first embodiment according to the present invention,
FIG. 2 shows an operation chart of the one cycle.

In FIG. 1, the DC input voltage Vin is the input terminals 1, 2
A load L having an output current of Io is connected to the DC output terminals 3 and 4. In addition, the inductor for resonance
The relationship of L1 << L2 and C1 << C2 is established between L18 and the load side choke coil L2 12, and between the resonance capacitor C19 and the load side capacitor C2 13 that smooths the output. First, when the switching element SW5 is turned on, the circuit current passing through the switching element SW5 passes through the resonance inductor L18 and the first diode D16 and the circuit current iL1.
As a result, Vin / L1 begins to flow with an inclination. Then, at time T1, it becomes equal to the output current Io, and the second diode D2 11
Cut off.

here At this point, the resonance inductor L18 and the resonance capacitor C19 form a series resonance circuit, and the circuit current iL1
Starts series resonance operation as shown below.

And At time T2 after the lapse of time, the circuit current iL1 is again output current I
equal to o, and the terminal voltage V of the resonance capacitor C1
_C1 will be equal to 2 Vin.

At time T3, the circuit current becomes zero and the first
The resonance operation is stopped by the one-way function of the diode D16 of. Further, this period T3-T1 is shown as follows.

At this point, all of the current source flowing through the choke coil L212 becomes due to the residual charge of the resonance capacitor C19, and its terminal voltage V _C1 linearly drops to zero at time T5, and at the same time the second diode D2 conducts.

On the other hand, since the switching element SW is in the ON state during this period, the current iL1 flowing through the resonance inductor L1 from the time T4 when the terminal voltage V _C1 of the resonance capacitor C19 falls below the input voltage Vin. As gradually increases.

Further, since the circuit current iL1 performs series resonance operation centered on the output current Io, if the maximum value of the output current is set to Io (Max), iL1 reaches the maximum current after turning on the switching element. The period tmax is shown as follows.

Therefore, if the ON period of the switching element SW is fixed within the range of the tmax value in this circuit, the switching element SW is turned off at time T6. Then, by turning off the switching element, the time T4
The circuit current iL1 that gradually increases later via the resonance inductor L1 loses its current source.

Therefore, in the conventional technique, the current becomes discontinuous, and a fatal defect that an excessive surge voltage is generated at both ends of the resonance inductor L18 at every turn-off occurs.

On the other hand, in the switching power supply circuit according to the present invention, when the switching element SW5 is turned off and the current source of the circuit current iL1 disappears, the third diode D3 10 becomes conductive and the input and output retrace lines are returned. After securing a current path between and, the circuit current gradually decreases and self-extinguishes. Therefore, it is clear that the effect of the present invention is great in that the generation of the surge voltage as in the conventional example described above can be prevented and the current flowing through the inductor L1 can be regenerated to the negative electrode side.

Further, even when the output current Io is small and the value at the time T3 is small, the falling slope of the terminal voltage V _C1 of the capacitor C1 becomes gentle, so that the time difference between the time T4 and the time T6 is not so large.

Therefore, the maximum value of the circuit current iL1 during this period is the maximum amplitude during the resonance operation period. It is clear that the value is sufficiently smaller than that of, and its switching loss can also be made sufficiently small.

From the above, in the switching power supply using the series resonance according to the present invention, even if the switching power supply in which the ON period of the switching element SW5 is fixed within the tmax and only the operation cycle is adjusted is simply configured, It is possible to operate with a low loss without increasing the generation of such noise.

FIG. 3 shows a second embodiment using the series resonant circuit according to the present invention.

In this embodiment, a DC input voltage is connected to the input terminals 111 and 112 to connect the primary winding N1 of the transformer T1 and the switching element SW5.
Is different from the first embodiment only in that a series circuit is connected to the secondary side winding N2 to generate an alternating palace voltage according to the on / off time of the switching element. At time t = 0 in FIG. 2, the switching element is turned on to output a forward pulse voltage in accordance with the transformation ratio between the primary winding and the secondary winding of the transformer, and By configuring so as to output a pulse in the reverse direction at time t = T6, the same operation and effect as in the first embodiment can be obtained. Therefore, the description of the operation is omitted.

〔The invention's effect〕

As described above, by using the switching power supply circuit according to the present invention, the resonance inductor, such as the conventional example, which occurs when the ON period of the switching element is fixed and only the operation cycle of the switching element is adjusted is simple. It is possible to prevent the generation of excessive noise due to the discontinuity of the current flowing through, and it is possible to operate with low loss.

Note that the resonance inductor in the present invention is not necessarily limited to the lumped constant, and naturally includes the inductance of the wiring, the leakage inductance of the coil component, and the resonance capacitor.
You may use the stray capacitance by wiring or winding.

[Brief description of drawings]

FIG. 1 is a switching power supply circuit diagram of a first embodiment according to the present invention. FIG. 2 is an operation chart of the switching power supply shown in FIG. FIG. 3 is a switching power supply circuit diagram showing a second embodiment of the present invention. FIG. 4 is a conventional switching power supply circuit diagram of this kind. FIG. 5 is an operation chart of the switching power supply circuit shown in FIG. 1, 2, 111, 112 ... Input terminals. 3, 4 ... Output terminals. 5 ... Switching element SW. 6 ... First diode D1. 8 ... Resonance inductor L1. 9 ... Resonance capacitor C1. 10 ... Third diode D3. 11 ... Second diode D2. 12… Load side choke coil L2 13… Load side capacitor C2. 14… Direction of output current Io. 15 ... Trance. Vin ... DC input voltage. L ... load. N1 ... Primary winding. N2 ... Secondary winding. Io ... Output current. iL ₁ … Circuit current.

Claims (2)

[Claims] 1. A switching power supply circuit for converting a DC input into alternating pulses using a switching element, directly or via a transformer, and rectifying and smoothing to obtain a DC output. A series circuit with an inductor for, and the inductor and a resonance capacitor are connected in series to form a unidirectional series resonance circuit,
In parallel to both ends of the resonance capacitor, the positive electrode of the second diode is connected in the positive direction of the power supply voltage, the negative electrode of the second diode is connected to the negative electrode of the power supply voltage, and the choke input of the choke coil and the smoothing capacitor is connected. A DC output is obtained by connecting a smoothing circuit, and the third
By connecting the positive electrode of the diode to the input side terminal of the resonance inductor and connecting the negative electrode of the third diode to the negative electrode of the power supply, even after the pulse voltage disappears,
A switching power supply circuit comprising a current path flowing through the resonance inductor. 2. The switching power supply circuit according to claim 1, wherein the switching element has a constant conduction time and a constant repetitive period, so that the output voltage is constant.